The goal of the proposed studies is to test the hypothesis that activation of fast twitch skeletal muscles is regulated by altering the phosphorylation state of the ryanodine receptor (RyR) sarcoplasmic reticulum (SR) calcium release channels. In this project the investigator will use the back phosphorylation-fragmentation-derivatization (BFD) and fragmentation Western blot (frag-blot) methods to determine the in vivo phosphorylation stoichiometries of specific sites in the different RyR isoforms in vivo in nonactivated muscles, as well as during tetanic stimulation, and during moderate and severe fatigue. Rat extensor digitorum longus (EDL) and frog semitendinosus muscles will be compared o ascertain if phosphorylation of the RyRs differ in muscles containing different ratios of RyR isoforms. In addition, selected studies will be expanded to include rabbit and chicken muscles to test for interspecies variations between mammalian and nonmammalian species.